MAP

Monday, 22 September 2014

Researchers at MIT's Laboratory for Nuclear Science have released new
measurements that promise to shed light on the origin of dark
matter.

The MIT group leads an international
collaboration of scientists that analyzed two and a half years' worth of data
taken by the Alpha Magnetic Spectrometer (AMS)—a large particle detector
mounted on the exterior of the International Space Station—that captures
incoming cosmic rays from all over the galaxy.

Among 41 billion cosmic ray events—instances of cosmic particles entering the detector—the
researchers identified 10 million electrons and positrons, stable antiparticles
of electrons. Positrons can exist in relatively small numbers within the cosmic
ray flux.

An excess of these particles has been observed by previous
experiments—suggesting that they may not originate from cosmic rays, but come
instead from a new source. In 2013, the AMS collaboration, for the first time,
accurately measured the onset of this excess.

The new AMS results may ultimately help scientists narrow in on the
origin and features of dark matter—whose collisions may give rise to
positrons.

The team reports the observed positron fraction—the ratio of the number
of positrons to the combined number of positrons and electrons—within a wider
energy range than previously reported. From the data, the researchers observed
that this positron fraction increases quickly at low energies, after which it
slows and eventually levels off at much higher energies.

The team reports that this is the first experimental observation of the
positron fraction maximum—at 243 to 307 gigaelectronvolts (GeV)—after half a century of cosmic ray experiments.

"The new AMS results show unambiguously that a new source of
positrons is active in the galaxy," says Paolo Zuccon, an assistant professor of physics at MIT. "We do not
know yet if these positrons are coming from dark matter collisions, or from
astrophysical sources such as pulsars. But measurements are underway by AMS
that may discriminate between the two hypotheses."

The new measurements, Zuccon adds, are compatible with a dark matter particle with mass on the order of 1 teraelectronvolt (TeV)—about 1,000 times the mass of a
proton.

Zuccon and his
colleagues, including AMS's principal investigator, Samuel Ting, the Thomas D.
Cabot Professor of Physics at MIT, detail their results in two papers published
today in the journal Physical Review Letters and in a third, forthcoming publication.

Catching a galactic stream

Nearly 85 percent of the universe is made of dark matter—matter that
somehow does not emit or reflect light, and is therefore invisible to modern
telescopes. For decades, astronomers have observed only the effects of dark
matter, in the form of mysterious gravitational forces that seem to hold
together clusters of galaxy that would otherwise fly apart. Such observations
eventually led to the theory of an invisible, stabilizing source of
gravitational mass, or dark matter.

The AMS experiment aboard the International Space Station aims to
identify the origins of dark matter. The detector takes in a constant flux of
cosmic rays, which Zuccon describes as "streams of the
universe that bring with them everything they can catch around the
galaxy."

Presumably, this cosmic stream includes leftovers from the violent
collisions between dark matter particles.

According to theoretical predictions, when two dark matter particles
collide, they annihilate, releasing a certain amount of energy that depends on
the mass of the original particles. When the particles annihilate, they produce
ordinary particles that eventually decay into stable particles, including
electrons, protons, antiprotons, and positrons.

As the visible matter in the universe consists of protons and electrons,
the researchers reasoned that the contribution of these same particles from
dark matter collisions would be negligible. However, positrons and antiprotons
are much rarer in the universe; any detection of these particles above the very
small expected background would likely come from a new source. The features of
this excess—and in particular its onset, maximum position, and offset—will help
scientists determine whether positrons arise from astrophysical sources such as
pulsars, or from dark matter.

After continuously collecting data since 2011, the AMS team analyzed 41
billion incoming particles and identified 10 million positrons and electrons
with energies ranging from 0.5 to 500 GeV—a wider energy range than previously
measured.

The researchers studied the positron fraction versus energy, and found
an excess of positrons starting at lower energies (8 GeV), suggesting a source for the particles other than the cosmic rays
themselves. The positron fraction then slowed and peaked at 275 GeV, indicating that the data may be compatible with a dark matter source
of positrons.

"Dark matter is there," Zuccon says. "We
just don't know what it is. AMS has the possibility to shine a light on its
features. We see some hint now, and it is within our possibility to say if that
hint is true."

If it turns out that the AMS results are due to dark matter, the
experiment could establish that dark matter is a new kind of particle, says
Barry Barish, a professor emeritus of physics and high-energy physics at
the California Institute of Technology.

"The new phenomena could be evidence for the long-sought dark
matter in the universe, or it could be due to some other equally exciting new
science," says Barish, who was not involved in the
experiments. "In either case, the observation in itself is what is
exciting; the scientific explanation will come with further experimentation.”